[libcxx] Don't use an undefined '+' in unsigned/octal/hexal print formats

[mstorsjo]

Summary

If building code like this:

unsigned long val = 1000;
snprintf(buf, sizeof(buf), "%+lu", val);

with clang, clang warns

warning: flag '+' results in undefined behavior with 'u' conversion specifier [-Wformat]

Therefore, don't construct such undefined format strings.

This fixes number formatting in mingw-w64 if built with
__USE_MINGW_ANSI_STDIO defined (there, the '+' flag causes a
leading plus to be printed when formatting unsigned numbers too,
while the '+' flag doesn't cause any extra leading plus in other
stdio implementations).

[• Quuxplusone]

Looks reasonable to me. FWIW, a more invasive rewrite would put the letter-computing code first:

    char letter = ((__flags & ios_base::basefield) == ios_base::oct) ? 'o' :
        ((__flags & ios_base::basefield) == ios_base::hex) ? 'x' :
        __signd ? 'd' : 'u';
    if (letter == 'd' && (__flags & ios_base::showpos))
        *__fmtp++ = '+';
    if (__flags & ios_base::showbase)
        *__fmtp++ = '#';
    while (*__len)
        *__fmtp++ = *__len++;
    *__fmtp++ = letter;
}

(Notice the names don't have to be _Uglified in this file because it's not a header.)

libcxx/src/locale.cpp

4600–4603

Please parenthesize (__flags & ios_base::showpos) for clarity. (Otherwise I could imagine a compiler thinking you'd typoed & for &&.)

[Mordante]

SMGT, but I'd like some unit tests to test the new correct behaviour. I think @Quuxplusone's proposed change is more readable, but I don't object against your version.

[mstorsjo]

In D103444#2790915, @Quuxplusone wrote:

Looks reasonable to me. FWIW, a more invasive rewrite would put the letter-computing code first:

Yup, that'd work too, and it seems like others prefer that version too, so I guess I can rewrite it that way.

In D103444#2791182, @Mordante wrote:

SMGT, but I'd like some unit tests to test the new correct behaviour.

Hmm, so a test under libcxx/test/libcxx somewhere, with a local class subclassing std::__1::__num_put_base so that the protected method __format_int can be called, and then testing the different flag combinations?

[Mordante]

In D103444#2792065, @mstorsjo wrote:

In D103444#2791182, @Mordante wrote:

SMGT, but I'd like some unit tests to test the new correct behaviour.

Hmm, so a test under libcxx/test/libcxx somewhere, with a local class subclassing std::__1::__num_put_base so that the protected method __format_int can be called, and then testing the different flag combinations?

Hmm I see testing this isn't that trivial. In that case I'd rather see a buildbot as suggested on Discord. But that's out of the scope of this patch.
LGTM, modulo @Quuxplusone's parenthesize request.
It looks like the build failures aren't caused by this patch, but keep an eye out after landing this patch.

libcxx/src/locale.cpp

if (__grouping.size() != 0 && __g_end - __g > 1)
         }
     }
 }
 
 void
 __num_put_base::__format_int(char* __fmtp, const char* __len, bool __signd,
                              ios_base::fmtflags __flags)
 {
-    if (__flags & ios_base::showpos)
+    if ((__flags & ios_base::showpos) &&
+        (__flags & ios_base::basefield) != ios_base::oct &&
+        (__flags & ios_base::basefield) != ios_base::hex &&
+  __signd)
         *__fmtp++ = '+';
     if (__flags & ios_base::showbase)
         *__fmtp++ = '#';
     while(*__len)
         *__fmtp++ = *__len++;
     if ((__flags & ios_base::basefield) == ios_base::oct)
         *__fmtp = 'o';
     else if ((__flags & ios_base::basefield) == ios_base::hex)